There are known apparatuses for nitriding metal materials and ferroalloys, operating at increased pressures in stationary conditions. In them, the material to be nitrided is crushed and then poured into crucibles or vessels, which are heated in furnaces operating at increased pressures. After the conclusion of the nitriding process, the heating of the furnaces is cut-off and after the nitrided material has cooled down to a preset temperature, the pressure inside the furnaces is released to become equal to the atmospheric and the nitrided material is removed.
In other known apparatuses the material is nitrided in a molten state. This is performed inside melting furnaces, operating at increased pressure, and the molten material is nitrided by bottom blowing through the crucible.
There are also known apparatuses for nitriding materials in nonstationary conditions. These are pipe furnaces comprising internal rotating pipes, in the one end of which enters the material to be nitrided and from the other end of which comes out the already nitrided material. These apparatuses are not pressurized and operate in nitrogen medium with a pressure slightly higher than atmospheric.
The drawbacks of the apparatuses operating in stationary conditions lie in the cyclicity of the nitriding process, the great energy losses for initial heating of the furnace, the long duration of the cooling of the furnace and the material charge down to a preset temperature, and their low productivity. A further drawback is the difficult removal of the nitrided material from the vessels in which it has been nitrided because of its sintering caused by its volume expansion as a result of nitriding. The removal of the material from the crucibles or vessels after nitriding is effected manually in extremely bad hygienic work conditions.
The apparatuses for nitriding in nonstationary conditions have a higher productivity than those operating in stationary conditions, but in them the nitrogen consumption is very high because they are not pressurized and the percentage of taken-up nitrogen is low because of the low pressure at which these apparatuses operate.
A common drawback of the apparatuses operating in stationary, as well as in nonstationary conditions, lies in the necessity for the material to be nitrided to be previously decarbonized as well as crushed to the smallest possible size of the particles, since this influences considerably the percentage of taken-up nitrogen. The crushing of the materials leads to high costs because of their great hardness.
The drawbacks of the apparatuses for nitriding materials in molten state lie in the small percentage of taken-up nitrogen, the extremely long duration of the nitriding process, as well as in the impossibility of good control of the process, this resulting in the impossibility in many cases to achieve a repeatibility of the end results.